Death and danger at migratory stopovers: problems with “predation risk”

Dierschke (2003) recently published a paper entitled, ``Predation hazard during migratory stopover: are light or heavy birds under risk?¿¿ He measured the body condition of 11 species of passerine migrants depredated by feral cats and raptors at an offshore stopover site, and used these data to address two hypotheses: (1) predation risk is higher for heavy birds because it decreases the take-off ability, and (2) predation risk is higher for lean birds because their need to feed increases their exposure to predators. Dierschke found that lighter weight individuals were strongly disproportionately represented among the victims, and concluded that ``it seems that the importance of reduced escape performance caused by the carrying of fuel loads is overestimated and possibly not biologically significant in terms of predation risk¿¿. We found Dierschke¿s data and analysis interesting and exciting, and believe that fundamentally, we interpret his results much as he does: lighter birds are killed disproportionately because they spend more time feeding and/or do so in ``riskier¿¿ habitats. But we disagree with the inference that massspecific ``predation risk¿¿ is therefore of little ecological importance or biological significance. In this commentary we consider the reasoning behind his conclusions, focusing particularly on the meaning of ``predation risk¿¿.